Handling of elastic yarn or thread, such as yarn or thread having a rubberized or other elastic core, or thread which is inherently and only made of elastic material, causes difficulties since the thread should be pulled off a supply spool with essentially uniform tension, yet without stretching the thread. Due to the elasticity of elastomeric threads, it is not possible to merely pull such threads off a supply spool which is, for example, rotatably located on a vertical spindle. The pull-off spools must be driven so that the thread pull-off will not subject the thread being pulled off to substantial tension and, further, to provide for uniformity of tension as the thread is being supplied.
Various types of thread supply apparatus for elastomeric thread have been proposed--see, for example, U.S. Pat. No. 3,590,601; British No. 1,442,573; German Patent Disclosure Document DE-OS No. 1 760 504. Thread supply apparatus of these types have a spool holder mandrel or pin which, usually, is vertically oriented and rotatable about a vertical axis of rotation. The thread supply spool, secured to the pin, is spring-pressed to a drive element which, for example, is formed as a cylindrical roller, and frictionally coupled with the circumference of the thread supply spool. Rather than using a roller, it is possible to engage a portion of the circumference of the supply spool, or of a plurality of spools, with a belt passing thereabout which is guided by suitable guide rollers, and driven at an appropriate speed, as well known.
Thread supply for a circular knitting machine usually employs an endless drive belt or tape which operates in a horizontal plane and drives various thread supply elements which are rotatable about vertical drive axes. The belt or tape is coupled with associated belt pulleys.
Placing spools on the respective holder pins or mandrels is comparatively complex and time-consuming, particularly if the thread is to be changed. It is not readily possible to associate a plurality of spools, for example containing threads of different colors, with one supply apparatus, although it would be advantageous to do so.
Belt as well as rim or circumferential drives have an additional disadvantage: As the thread is being used, the frictional engagement between the drive rollers or drive belts, and the cylindrical drive element itself, becomes less, so that it is necessary to press the spool and the drive element against each other with increasing force in order to prevent undesirable slippage therebetween.
Similar considerations apply to supply apparatus for elastomeric threads as known in the art, in which a common drive roller is provided for a plurality of pull-off spools, all positioned in a common horizontal plane. The drive roller is driven by a continuously variable speed drive and relative pressure of the drive roller with respect to the spools is effected. The spools are secured to pins or mandrels which are pivotable on pivot arms which, in turn, are movable about a common axis, and are biassed in direction towards the drive roller by spring pressure.